Recently, surface acoustic wave (SAW)-based micro sensors are generally used for tire-pressure monitoring systems, temperature sensors, biosensors, and environmental gas sensors. SAW-based micro sensors have more merits than semiconductor-based micro sensors.
First, since an SAW is generated on a surface of a micro sensor by using radio frequency (RF) energy wirelessly supplied from the outside and variances in a time, a phase, and an amplitude of the SAW reflected by a reflector and returns, a battery is not necessary.
Second, since it is not necessary to equip a semiconductor integrated circuit (IC) chip, it is possible to maintain stable properties for a long time in a rough and poor environment such as a high temperature, humidity, and impacts. Third, long distance wireless communication is possible with RF energy supplied from the outside. Fourth, in a quickly rotating and moving range such as in a tire of a vehicle, it is possible to sense factors in real time.
A measurement of variances in environmental elements by using SAW-based micro sensors may be divided into two ways. One is a method of measuring a variance of a resonance frequency of an SAW-based micro sensor caused by an environmental element, and another is a method of measuring a variance of a reflection peak reflected by an SAW-based micro sensor and returning.
Among them, since the method of measuring the variance of the resonance frequency of the SAW-based micro sensor needs an inter-digital transducer (IDT), there is a limitation in wireless measurements. Accordingly, the method of measuring the variance of the reflection peak reflected and returning is appropriate for wireless measurements. Such variances of reflection peaks may be measured by using network analyzers. However, since network analyzers are expensive and have large sizes, there is a limitation in being actually applied to industrial fields. Accordingly, it is necessary to provide small sized cheap wireless measurement devices.
Such small sized cheap wireless measurement devices employ a method of measuring variances of environmental elements by measuring time delays of SAWs returning from SAW-based micro sensors. However, to measure very short time delays, that is, to measure minute variances in environmental elements, in other words, to have high resolution, small sized cheap wireless measurement devices are equipped with expensive elements capable of generating clocks of several GHz-frequency. Due to this, prices of wireless measurement devices using time delays are increased.